The present invention relates to an optical gain fiber; and, more particularly, to an optical gain fiber doped with rare earth ions for improving gain efficiency thereof.
As a thulium ion, usefully used in an optical communication, is an active ion excited easily by a laser diode in common use, a thulium-doped optical amplifier is implemented by using a fluorescence generated from a transition of a thulium ion. To obtain a high gain in 1.4 xcexcm band by using the thulium-doped optical amplifier, many researches for fluoride glass and tellurite glass are being advanced.
In the optical gain fiber amplifier using the near-infrared fluorescence in 1.4 xcexcm band generated from a thulium-doped (Tm3+, ) amorphous material, the fluorescence in a 800 nm band, in 1,450 nm band and in 2,300 nm band are emitted simultaneously from a 3H4 level which absorbs a photon energy in 800 nm band. In this case, the intensity of the fluorescence is considerably affected by a branching ratio. The branching ratio of the 1,4 xcexcm band is lower approximately 10% than the fluorescence in 800 nm band, whereby most of electrons, excited to the 3H4 level, are transited to a 3H6 level in emitting the fluorescence in 800 nm band. Therefore, the efficiency of an amplification in 1.4 xcexcm band, is decreased in the thulium-doped amplifier owing to an amplified spontaneous emission (ASE) or lasing of the fluorescence in the 800 nm band.
As is well known, an optical amplifier has become a major device not only for constructing an optical transmission system, but also for implementing an optical network. Hence, various fiber amplifiers have been developed for the transmission windows of the conventional single mode silica fibers ranging from 1.2 xcx9c1.6 xcexcm.
With regard to the 1.4 xcexcm band, thulium (Tm3+)-doped fiber amplifiers have been proposed, based on the stimulated transition from 3H4 (upper level) to 3F4 (lower level). The amplification of these Tm3+-doped fiber amplifiers is limited by the fact that the lifetime of the upper level is shorter than that of the lower level, which results in a difficulty in forming a population inversion. Therefore, the lower level needs to be depopulated to construct an efficient amplifier with Tm3+-doped fibers.
Recently, several methods have been proposed for depopulating the Tm3+lower level.
One of these methods is that holmium ions (Ho3+) have been proposed as effective codopant ions. By codoping Ho3+ions with Tm3+ions in a core of an optical gain fiber, the lower level of Tm3+is depopulated through the energy transfer process. Ho3+ions depopulate the lower level effectively, and do not depopulate the upper level of Tm3+, very much by cross relaxation, so efficient amplification can be expected.
However, the above-described method still has a problem of an amplified spontaneous emission (ASE), arising from the 800 nm transition of Tm3+.
It is, therefore, an object of the present invention to provide an optical gain fiber doped with rare earth ions for improving a gain efficiency of a fiber amplifier operating at the 1,450 nm band by inhibiting an amplified spontaneous emission at 800 nm.
In accordance with one aspect of the present invention, there is provided an optical gain fiber for amplifying an optical signal, comprising: a core doped with a first rare-earth ion in a portion thereof for amplifying the optical signal; and a clad doped with a second rare earth ion for absorbing an amplified spontaneous emission (ASE) emitted from the first rare earth ion, wherein the portion of the core and the potion of the clad are separated by the remaining portions of the core and the clad.